The present invention generally relates to a differential scanning calorimeter and, in particular, relates to such a calorimeter having means for simultaneously correcting for linear and curvilinear baseline variations.
In general a differential calorimeter is a sophisticated analytical instrument which measures the thermal characteristics of a sample material. Specifically, a sample and a reference are controllably heated over time and the temperature of each monitored. The thermal characteristics of the reference are known and, preferably, the reference is chosen from a material which does not undergo a transformation during the analysis. Thus, when the sample undergoes a transformation, such as sublimation, boiling, or the like, that transformation is clearly discernable. By knowing the temperature at which the transformation occurs as well as the energy either absorbed or expended during the transformation the sample material can be rather accurately characterized.
In one type of calorimeter the sample and reference materials are placed in holders positioned within a block of thermally conductive material. The block is heated during analysis whereby the holders are subjected to substantially identical thermal conditions. Assuming that the block has a substantially uniform structure throughout, i.e. there are no air pockets or significant molecular structural defects, the heat transfer characteristics of the holders are effectively identical. Hence, the baselines of the thermographs taken are quite flat. The major drawback of this type of calorimeter is that, because of the large mass of the block, considerable time is lost before each analysis since all dynamic thermal gradients within the block must first be removed and a steady state maintained.
In another type of differential scanning calorimeter the sample holder and the reference holder are separately heated via individual dedicated heaters. Thus, the mass to be heated, i.e. only the holders, is significantly reduced resulting in an increased number of analysis during a comparable period of time. However, this arrangement introduces both linear and curvilinear changes in the measured baseline. These changes are due to the differences between the inherent heat loss characteristics of individual holders.
As a consequence, sample and reference holders for use in such an instrument are usually closely matched by the manufacturer and sold as a pair. Nevertheless, in a modern differential scanning calorimeter, the thermal differences between even the most carefully matched pair of holders results in measurable baseline changes. Conventionally, these changes can be separated into a linear change and a curvilinear change. Historically, in order to correct for these baseline changes an operator performed a two step procedure, first correcting for the curvilinear change and then correcting for the linear change. In order to improve the accuracy of the results of an analysis these steps would be repeated at least once. Clearly, this two step procedure can be time consuming although fortunately, this correction procedure needs to be performed relatively infrequently.